1. Field of the Invention
The present invention relates to a proton conductor in which a polymer is bonded to pores of a porous body, and a method for producing the same.
2. Description of the Related Art
In recent years, it has been attempted to use a proton conductor as an electrolyte for a fuel cell. In this case, all parts of the fuel cell may be constructed from solid materials, because the proton conductor itself is a solid. Therefore, the structure is simplified. Further, no liquid leakage occurs, and hence the frequency of required maintenance operations can be remarkably reduced.
One known type of proton conductor includes an organic material, which has a functional group, such as a phosphoric acid group or a sulfonic acid group, bonded by substitution to pores of an organic porous body. For example, Japanese Laid-Open Patent Publication No. 2002-83612 describes a proton conductive polymer as an organic material, which is obtained in pores by mutually polymerizing monomers, each of which exhibits proton conductivity. Japanese Laid-Open Patent Publication No. 2002-83514 and International Patent Publication WO 00/54351 describe a proton conductive polymer that is bonded or charged to the pores of a porous body, in order to provide a proton conductive polymer within the pores.
Proton conduction in such a proton conductor is caused by successive occurrence of a phenomenon, in which a proton is dissociated from the functional group, such as a sulfonic acid group, existing at the terminal end, and wherein the proton is bonded by substitution to a proton existing on another functional group.
In the inventions described in Japanese Laid-Open Patent Publication Nos. 2002-83612 and 2002-83514 and International Patent Publication WO 00/54351, it has been postulated that the polymer, which is obtained in the pores, constitutes a cross-linked material, in which principal chains are three-dimensionally bonded to one another, as shown in FIG. 11. Such a cross-linked material becomes highly rigid because of the three-dimensional bonding. Therefore, it is difficult to subject the cross-linked material to oscillation or fluctuation. In the case of the cross-linked material, which is adverse to oscillation or fluctuation, it is difficult for the functional groups to approach one another when the spacing distance between the functional groups, such as sulfonic acid groups, is large. Therefore, proton mobility and proton conductivity are decreased.